1. Field of the Invention
The present invention relates to an objective lens actuator for performing focusing control or tracking control in an optical pickup device used in a compact disk player, an optical video disk player, an optical disk device, and the like.
2. Related Background Art
When a beam emitted from a light source is to be focused in a mode for optically recording information from an optical disk or reproducing it from the disk, the actuator performs so-called focus control so that an in-focus point of a focused beam (to be referred to as a beam spot hereinafter) traces a disk recording surface in accordance with changes in surface level of the disk. At the same time, the actuator performs so-called tracking control so that the beam spot traces recording tracks in accordance with eccentricity of the recording tracks.
When a target track is to be accessed, an optical head is moved by a linear motor to a predetermined position, and an objective lens position is then finely adjusted.
In order to perform the above control operations, the actuator comprises a means for driving an objective lens in a direction of thickness of the optical disk (i.e., an optical axis of the objective lens) and a means for driving the objective lens in a radial direction of the optical disk (i.e., a direction perpendicular to the optical axis) to perform tracking control or fine adjustment for accessing the target track.
A conventional example of an actuator having the above arrangement is shown in FIG. 1. An objective lens 1 is fixed in a lens holder 3. The lens holder 3 is held on a support shaft 6 at the central portion of the lens holder 3 so as to be slidable in the axial direction and pivotal in the direction of rotation. The shaft 6 is mounted on a base (not shown). A balance weight 2 is mounted in the lens holder 3 to be axially symmetrical with the objective lens 1, so that the center of gravity of the lens holder 3 is located at the axial center of the support shaft 6 (see FIG. 2). A focusing coil 4 is wound around the outer circumferential surface of the lens holder 3. Tracking coils 5a, 5b, 5c, and 5d are mounted in the peripheral portion of the lens holder 3 at predetermined angular intervals. In order to generate a magnetic field crossing the coils 4, 5a, 5b, 5c, and 5d , magnets 7a and 7b are located on both sides of the lens holder 3. The magnets 7a and 7b are supported on outer yokes 8a and 8b. Inner yokes 9a and 9b are continuous with the outer yokes 8a and 8b on their bottom surfaces. The inner yokes 9a and 9b are inserted into through holes 12a and 12b formed in the lens holder 3.
During focusing control, when a current is supplied to the focusing coil 4 in a clockwise or counterclockwise direction, an electromagnetic force acts in a direction of the support shaft 6 in a gap formed between the left and right magnetic circuits (constituted by the magnets 7a and 7b, the outer yokes 8a and 8b, and the inner yokes 9a and 9b). The objective lens 1 is axially moved in correspondence with a change in surface level of the optical disk, so that the beam spot follows the recording surface of the disk.
During tracking control, when a current is supplied to each of the tracking coils 5a, 5b, 5c and 5d connected in series, in one direction or in the reverse direction thereof, an electromagnetic force acts about the support shaft 6 as rotation couple forces in a gap formed between the left and right magnetic circuits, so that the beam spot traces the recording tracks in accordance with eccentricity of the tracks.
In the above conventional arrangement, however, when the lens holder 3 which holds the objective lens 1 is to be driven in the focusing or tracking direction, these movable actuator components such as the lens holder 3 resonate at a natural resonant frequency thereof, thereby adversely affecting servo control. A sub-resonance occurring at a high frequency range of several hundreds of Hz to several tens of kHz causes degradation in accurate positioning (tracking of the beam spot for the changes in surface level of the disk and eccentricity of tracks) of the objective lens 4 and disables control.